Actuator units with a piezoelectric actuator are in widespread use and are more and more used for fuel injectors for internal combustion engines. They have the advantage of having very fast response timed actuating signals and therefore enable multiple injections into a cylinder of the internal combustion during one working cycle of the cylinder. Essential for a reliable operation of a fuel injector with an actuator unit with a piezoelectric actuator is, that the piezoelectric actuator is precisely pretensioned.
WO 00/08353 discloses an actuator unit with a tube spring, a piezoelectric actuator, that is inserted into the tube spring, a first cap, that is connected to the tube spring at a first free end of the tube spring and which is adjoined by the piezoelectric actuator and a second cap, that is connected to the tube spring at a second free end of the tube spring and which is also adjoined by the piezoelectric actuator. The first and second caps are welded to the tube spring. The tube spring is preferably made out of steel and has recesses that are formed in a bone-shaped manner. The piezoelectric actuator is pretensioned by the tube spring. The pretension forces are transmitted to the piezoelectric actuator by the first and second cap. The piezoelectric actuator is pretensioned with a force of 800 to 1000 Newton. When manufacturing the actuator unit the piezoelectric actuator needs to be inserted into the tube spring after the first cap is welded to the tube spring and before the second cap is welded to the tube spring. While welding the second cap to the tube spring the given pretension force must be applied to the second cap. This means that a tool needs to contact the second cap in a respective manner.